NSW Health Director of Health Protection Jeremy McAnulty said on Wednesday people who have had diarrhoea should stay out of public swimming pools for two weeks.

"If you've had crypto and your symptoms resolve you can still expel them in your faeces and some of that can stick to your skin around the bottom and get washed off in the pool when you're swimming," Dr McAnulty said.

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"We've seen a sharper increase in the last couple of weeks than we've had in recent years and we might be on the upswing of something big, so if we can avoid it getting in swimming pools we can avert a chain outbreak. We're trying to get in first."

The parasite was so hardy that it could survive the regular chlorine levels for swimming pools and so small that it could evade filtration systems, he said.

Cases have been notified in the Hunter, New England, southern parts of the state and northeastern parts of Sydney, though no swimming pools have yet been identified as the source of infection.

It causes people to have 10 to 20 watery stools per day and can last several weeks to several months.

Mostly it affects children under the age of five, who are more likely to swallow water while swimming and have lower levels of immunity.

At the biggest outbreak in 1998, more than 1000 cases were confirmed, and swimming pools along the east coast were found to be contaminated.

NSW Health has advised people to avoid infection by washing their hands after visiting the toilet, changing nappies or handling animals, to avoid swallowing pool or spa water and to boil untreated water before drinking it while camping or hiking.